treewide: Use fallthrough pseudo-keyword

Replace the existing /* fall through */ comments and its variants with
the new pseudo-keyword macro fallthrough[1]. Also, remove unnecessary
fall-through markings when it is the case.

[1] https://www.kernel.org/doc/html/v5.7/process/deprecated.html?highlight=fallthrough#implicit-switch-case-fall-through

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

net/sctp/ipv6.c

@@ -875,7 +875,7 @@ static int sctp_inet6_af_supported(sa_family_t family, struct sctp_sock *sp)
 	case AF_INET:
 		if (!__ipv6_only_sock(sctp_opt2sk(sp)))
 			return 1;
-		/* fallthru */
+		fallthrough;
 	default:
 		return 0;
 	}